The present invention relates to a Penning vacuum meter comprising a cathode and an anode.
A Penning vacuum meter is a cold cathode vacuum meter, the operation of which is based on a cold discharge. It comprises a tube with two non-heated electrodes, cathode and anode, between which a discharge is ignited and maintained by means of a DC voltage (about 2 kV during operation), whereby said discharge is maintained in a steady state also at very low pressures. This is attained by making the path for the electrons so long with the aid of a magnetic field, that their collision rate with the gas molecules is sufficiently high in order to form the required quantity of charge carriers to maintain the discharge.
In the tubes of Penning vacuum meters, the cathode material is disintegrated by the ions which are accelerated in the direction of the cathode, particularly so in connection with coating processes involving heavy noble gases (argon, xenon) being performed in the pressure range between 10.sup.-4 and 10.sup.-2 mbar. The disintegrated cathode material forms on the walls of the measurement tube a conductive layer.
Generally, stainless steel is employed as the material for the cathode. Owing to the cathode disintegration process described ("sputtering") the service life of the cathode is limited. Moreover, disintegrated stainless steel cathode material is magnetic. Parts coming loose from the formed layers are capable of aligning themselves in the magnetic field thereby causing short circuits. In addition, parts of the measuring cell, which actually should be electrically isolating (ceramic feedthroughs) may become electrically conducting. These disadvantages result in an unstable readout or extinguishing of the discharge and thus in a failure of the measurement tube. Even so, the Penning vacuum meter is employed very often, since it is cost-effective and insensitive to air inrushes and vibrations.